code frames

compiler/parse/index.js

@@ -2,6 +2,41 @@ import { locate } from 'locate-character';
 import fragment from './state/fragment.js';
 import { whitespace } from './patterns.js';
 import { trimStart, trimEnd } from './utils/trim.js';
+import spaces from './utils/spaces.js';
+
+function tabsToSpaces ( str ) {
+	return str.replace( /^\t+/, match => match.split( '\t' ).join( '  ' ) );
+}
+
+function ParseError ( message, template, index ) {
+	const { line, column } = locate( template, index );
+	const lines = template.split( '\n' );
+
+	const frameStart = Math.max( 0, line - 2 );
+	const frameEnd = Math.min( line + 3, lines.length );
+
+	const digits = String( frameEnd + 1 ).length;
+	const frame = lines
+		.slice( frameStart, frameEnd )
+		.map( ( str, i ) => {
+			const isErrorLine = frameStart + i === line;
+
+			let lineNum = String( i + frameStart + 1 );
+			while ( lineNum.length < digits ) lineNum = ` ${lineNum}`;
+
+			if ( isErrorLine ) {
+				const indicator = spaces( digits + 2 + tabsToSpaces( str.slice( 0, column ) ).length ) + '^';
+				return `${lineNum}: ${tabsToSpaces( str )}\n${indicator}`;
+			}
+
+			return `${lineNum}: ${tabsToSpaces( str )}`;
+		})
+		.join( '\n' );
+
+	this.message = `${message} (${line + 1}:${column})\n${frame}`;
+	this.loc = { line, column };
+	this.shortMessage = message;
+}
 
 export default function parse ( template ) {
 	const parser = {
@@ -13,9 +48,12 @@ export default function parse ( template ) {
 			return this.stack[ this.stack.length - 1 ];
 		},
 
+		acornError ( err ) {
+			parser.error( err.message.replace( /\(\d+:\d+\)$/, '' ), err.pos );
+		},
+
 		error ( message, index = this.index ) {
-			const { line, column } = locate( this.template, index );
-			throw new Error( `${message} (${line}:${column})` );
+			throw new ParseError( message, this.template, index );
 		},
 
 		eat ( str, required ) {

compiler/parse/read/expression.js

@@ -1,11 +1,15 @@
 import { parseExpressionAt } from 'acorn';
 
 export default function readExpression ( parser ) {
-	const node = parseExpressionAt( parser.template, parser.index );
-	parser.index = node.end;
+	try {
+		const node = parseExpressionAt( parser.template, parser.index );
+		parser.index = node.end;
 
-	// TODO check it's a valid expression. probably shouldn't have
-	// [arrow] function expressions, etc
+		// TODO check it's a valid expression. probably shouldn't have
+		// [arrow] function expressions, etc
 
-	return node;
+		return node;
+	} catch ( err ) {
+		parser.acornError( err );
+	}
 }

compiler/parse/read/script.js

@@ -1,10 +1,5 @@
 import { parse, tokenizer } from 'acorn';
-
-function spaces ( i ) {
-	let result = '';
-	while ( i-- ) result += ' ';
-	return result;
-}
+import spaces from '../utils/spaces.js';
 
 export default function readScript ( parser, start, attributes ) {
 	const scriptStart = parser.index;

compiler/parse/utils/spaces.js

@@ -0,0 +1,5 @@
+export default function spaces ( i ) {
+	let result = '';
+	while ( i-- ) result += ' ';
+	return result;
+}